The illumination of the telephone dials of premium telephone sets is presently achieved by the use of an incandescent lamp that is powered from a 110 volt ac source on the subscriber's premises. The low voltage power for operating the lamp is derived by means of a voltage reducing transformer that is plugged into an ac receptacle of the subscriber. The transformer is connected by a power cord to a telephone junction box, and a nonstandard six conductor line cord completes the connection to the telephone set.
The foregoing components and their installation result in increased costs to the telephone company over that involved in providing a non-illuminated set. Furthermore, inasmuch as incandescent lamps have a limited life, there is the likelihood of the telephone company having to periodically send a serviceman to the subscriber's residence to replace the lamp, resulting in an additional significant cost. Finally, the connection of a 110 volt ac source to the telephone set, even though buffered by a step down transformer, introduces a possible hazard to the user as well as to the individual who must install and service the telephone set.
In view of the foregoing there has been an interest in replacing the incandescent lamp by an illumination source that has a much longer life and that can be energized by the low voltage dc power on the telephone line.
One such arrangement is described in U.S. Pat. No. 3,132,211 issued to E. E. Bauman on May 5, 1964. Bauman discloses a telephone dial light comprising an electroluminescent cell positioned beneath the number plate of a rotary dial. Since an electroluminescent cell requires ac excitation, Baumen's illumination circuit, which is connected across the telephone line conductors in parallel with the speech network, includes an inverter for converting the dc voltage of the telephone line to an ac voltage of a value to energize the cell. Thus, this arrangement has the disadvantage of necessitating an ac power supply circuit. It also has the disadvantage of again introducing ac potentials as high as 100 volts into the telephone set.
An arrangement for replacing the incandescent lamp which does not incur these disadvantages is described in U.S. Pat. No. 3,784,760 issued to R. M. Rickert on Jan. 8, 1974. Rickert discloses a telephone illumination circuit including a light emitting diode (LED) as a dial illumination source, the illumination circuit being connected across the telephone line conductors in parallel with the speech network. Power for energizing the LED is derived from the telephone line, but in order to insure that adequate power is available on long telephone loops, energy storage means in the form of a battery is included in the illumination circuit. The battery is connected so as to receive a steady trickle charge from the central office dc power applied to the telephone line, and when the set goes off-hook, the local battery is applied to the LED to provide the desired illumination. However, in order to prevent the battery from being discharged during the course of a long telephone call to the point that it is not capable of providing dial illumination for an immediately subsequent call, the illumination circuit further includes timing means for disconnecting the LED after a particular time interval.